Forced draft direct vent type room heater

ABSTRACT

The room heater has, in an outer case  1 , both a first air flow passage that leads to a vent hole  12  through a section in which the main heat exchanger  6  is arranged and a second air flow passage that does not pass through the section in which the main heat exchanger  6  is arranged but leads to the vent hole  12  through a section in which a supplementary heat exchanger  6′  is arranged provided. At least one of the first damper  20  which can freely block the first air flow passage and the second damper  21  which can freely block the second air flow passage is provided, and the first air flow passage or the second air flow passage is blocked in a weak combustion period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a forced draft direct vent type roomheater which forcibly supplies air for combustion and exhausts acombusted gas.

2. Description of the Related Art

Conventionally, such a room heater is known as is described in JapaneseUtility Model Laid-Open No. 55-94647. This room heater has a burnerhoused in a combustion housing provided in an outer case. In thecombustion housing, air for combustion is supplied and a combusted gasis exhausted by a combustion fan. In an exhaust path for exhausting thecombusted gas therethrough, a main heat exchanger is provided in theupper stream side, and a supplementary heat exchanger is provided in thedown stream side. In the outer case, an air intake and a vent hole areprovided. In the outer case, a warm air fan is provided, and this warmair fan takes in room air from the air intake and sends the air to theinside of the room from the vent hole through the main heat exchangerand the supplementary heat exchanger.

In the above described official bulletin, the supplementary heatexchanger is arranged in the back side of the main heat exchanger. Theroom air flows to the vent hole through an air flow passage in the backside and an air flow passage in the front side of the main heatexchanger. Thereby, the room air flowing in the air flow passage in theback side is heated by the main heat exchanger and the supplementaryheat exchanger.

In addition, a damper which can freely block the flow of the room air isprovided in the air flow passage in the back side in the inner part ofthe outer case. The damper passes the room air to both of the air flowpassages in the front side and the back side, when the length of anexhaust extension tube connected to the downstream end of the exhaustpath is short. When the length of the exhaust extension tube is long,the damper blocks the flow of the room air flowing into the air flowpassage in the back side. This aims at lowering the heat exchangeefficiency of the room heater itself and raising the temperature of thecombusted gas flowing into the exhaust extension tube, because when theexhaust extension tube provided in the room is long, a heat releasedfrom the exhaust extension tube also contributes to heating of theinside of the room.

By the way, a room heater also is recently required to provide higherefficiency as a measure against global warming. Here, in the abovedescribed conventional example, even when the length of the exhaustextension tube is long, the heat exchange efficiency is improved betterby a method of passing room air to both of the air flow passages in thefront side and in the back side. Accordingly, regardless of the lengthof the exhaust extension tube, it is advantageous for increasing theefficiency to pass the room air through both of the air flow passages inthe front side and in the back side.

However, in the above described conventional example, the room airflowing into the air flow passage in the back side of the main heatexchanger results in being heated by the supplementary heat exchanger ina state of having been heated by the main heat exchanger. For thisreason, the heat exchange efficiency of the supplementary heat exchangeris not enhanced so much.

In addition, there is occasionally employed a structure of providing adrain dish for making a drain formed in the exhaust extension tube flowinto itself in the outer case, and passing the room air (warm air) whichis heated by the heat exchanger to a section in which the drain dish isarranged to evaporate the drain in the drain dish. In this case, itcauses the following problem when the heat exchange efficiency isenhanced. That is, in a weak combustion period, the temperature of thecombusted gas flowing in the exhaust extension tube is excessivelylowered, so that an amount of the formed drain exceeds an amount of anevaporated drain, and the drain overflows from the drain dish. In orderto prevent the overflow of the drain from the drain dish, it isnecessary to shorten the length of the exhaust extension tube and toreduce the amount of the formed drain. As a result, a position where theroom heater can be set becomes limited.

SUMMARY OF THE INVENTION

The present invention is designed with respect to the above describedpoints, and its object is to provide a forced draft direct vent typeroom heater which can enhance its heat exchange efficiency as much aspossible, and can prevent the drain from overflowing from the drain dishin the weak combustion period even without shortening the length of theexhaust extension tube.

The present invention provides a forced draft direct vent type roomheater comprising: an outer case, a combustion housing which is providedin the outer case and houses a burner therein, a combustion fan forforced draft direct ventilation, which is provided on the way of any oneof an air supply path that supplies air for combustion to the combustionhousing and an exhaust path that exhausts a combusted gas from thecombustion housing, a main heat exchanger in an upstream side and asupplementary heat exchanger in a downstream side, which are provided inthe outer case and are provided on the way of the exhaust path, an airintake and a vent hole which are opened in the outer case, a warm airfan in the outer case for taking in room air from the air intake andsending the air to the inside of the room from the vent hole through asection in which the main heat exchanger and the supplementary heatexchanger are arranged, and a drain dish which is provided in the outercase and makes a drain formed in an exhaust extension tube that isconnected to a downstream end of the exhaust path to flow into the draindish itself, wherein a first air flow passage that leads to the venthole in the outer case through a section in which the main heatexchanger is arranged and a second air flow passage that does not passthrough the section in which the main heat exchanger is arranged but isled to the vent hole through a section in which the supplementary heatexchanger is arranged are separately formed in the outer case, at leastone of a first damper which can freely block the flow of the room airflowing into the first air flow passage due to the warm air fan and asecond damper which can freely block the flow of the room air flowinginto the second air flow passage due to the warm air fan is provided,and the flow of the room air flowing into the first air flow passage orthe second air flow passage is blocked, in a weak combustion period inwhich a combustion amount of the burner is a predetermined amount orless.

According to the present invention, the room air passed into the firstair flow passage is heated by the main heat exchanger, and the room airpassed into the second air flow passage is also heated by thesupplementary heat exchanger. Here, the room air passed into the secondair flow passage flows to a section in which the supplementary heatexchanger is arranged, in a state of being kept at a low temperaturewithout being heated by the main heat exchanger, and is efficientlyheated by the supplementary heat exchanger. Accordingly, the heatexchange efficiency of the whole room heater is enhanced as much aspossible.

In a weak combustion period, the flow of the room air flowing into thefirst air flow passage or the second air flow passage is blocked, sothat the heat exchange efficiency is prevented from lowering, and thetemperature of the combusted gas flowing in the exhaust extension tubeis prevented from being excessively lowered. Accordingly, it is possibleto effectively prevent the amount of a formed drain from exceeding anamount of an evaporated drain and the drain overflowing from the draindish in the weak combustion period, even without shortening the lengthof the exhaust extension tube.

In addition, in a forced draft direct vent type room heater according tothe present invention, it is preferable that both of the first damperand the second damper are provided therein, the first air flow passageis formed along the outer surface of the combustion housing, and whenthe temperature of the combustion housing is a predetermined temperatureor higher in the temperature-control OFF operation period, the room airis passed into the first air flow passage and the flow of the room airflowing into the second air flow passage is blocked, and when thetemperature of the combustion housing is lower than the predeterminedtemperature, the room air is passed into the second air flow passage andthe flow of the room air flowing into the first air flow passage isblocked.

Thereby, when the deviation of a room temperature from a set heatingtemperature is a predetermined value or more, the room air is heatedonly by the main heat exchanger, so that the temperature of thecombusted gas flowing in the exhaust extension tube can be preventedfrom being excessively lowered while a heating amount necessary forpromptly raising the room temperature to the set heating temperature ismaintained. In addition, when the deviation of the room temperature fromthe set heating temperature is less than the predetermined value, theroom air is heated only by the supplementary heat exchanger, so that theheating amount decreases, and the room temperature is inhibited fromovershooting to a temperature over the set heating temperature, whichenhances the temperature control performance.

By the way, when a heating load is light, the room temperature maycontinue rising even if the burner is burned at a lower limit combustionamount. Then, when the room temperature exceeds a predetermined upperlimit temperature which is set higher than the set heating temperature,the temperature-control OFF operation is conducted which rotates thewarm air fan at a low speed in a state in which the burner stopscombustion. The warm air fan is not stopped in the temperature-controlOFF operation period in order to keep a difference between upper andlower temperatures in the room as small as possible.

Here, when the temperature-control OFF operation is started, a warm airis sent from a vent hole because heat is accumulated in the room heater,but the heat in the room heater is released as time passes, so that thetemperature of the air sent from the vent hole is lowered, and acool-felt air starts to be sent. In this case, the timing at which thecool-felt air starts to be sent can be delayed by blocking the flow ofthe room air flowing into the first air flow passage or into the secondair flow passage in the temperature-control OFF operation period,because the heat in the room heater is slowly released.

When the first air flow passage is formed along the outer surface of thecombustion housing, if the temperature of the combustion housing is apredetermined temperature or higher in the temperature-control OFFoperation period, it is preferable to pass the room air through thefirst air flow passage and block the flow of the room air flowing intothe second air flow passage, and if the temperature of the combustionhousing is lower than a predetermined temperature, it is preferable topass the room air through the second air flow passage and block the flowof the room air flowing into the first air flow passage. Thereby, whenthe temperature of the combustion housing is the predeterminedtemperature or higher, the main heat exchanger and the combustionhousing release the heat, and when the temperature of the combustionhousing is lowered and reaches a temperature lower than thepredetermined temperature due to the heat release, the supplementaryheat exchanger releases the heat. Then, the timing at which thecool-felt air starts to be sent can be delayed as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a room heater according to an embodimentof the present invention, which is viewed from an upper side in adiagonally front right direction;

FIG. 2 is a perspective view of a room heater according to theembodiment, which is viewed from an upper side in a diagonally backsideleft direction;

FIG. 3 is a front view illustrating a state in which a front panel and afront door of a combustion housing in the room heater according to theembodiment are removed;

FIG. 4 is a sectional view taken along the line IV-IV of FIG. 3;

FIG. 5 is a flow chart illustrating the control in a temperature-controloperation period; and

FIG. 6 is a flow chart illustrating the control in a temperature-controlOFF operation period.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment in which the present invention is applied to a fireplacetype room heater will now be described below. This room heater comprisesan outer case 1 and a combustion housing 2 which is provided in theouter case 1, as is illustrated in FIG. 1 to FIG. 4. The outer case 1 isconstituted by a bottom plate 1 a, right-and-left side plates 1 b, aback plate 1 c, an upper plate 1 d, a front panel 1 e, a first extensioncase 1 f which is installed in a portion closer to one side of the backface of the back plate 1 c, and a second extension case 1 g which isinstalled from the other half side of the back face of the back plate 1c to the upper part of the first extension case 1 f.

A burner 3 and a pilot burner 3 a for igniting the burner 3 are arrangedin the lower part of the inside of the combustion housing 2. Inaddition, a burner supporting plate 2 a in which an opening for fittingthe upper end part of the burner 3 is formed is arranged in thecombustion housing 2, and a false firewood 4 which is formed from arefractory material such as ceramics is arranged on the burnersupporting plate 2 a. A glass plate 1 b is fitted in a front face of thecombustion housing 2, and a window 1 h through which the glass plate 2 bis overlooked is opened in the front panel 1 e of the outer case 1.Thus, the inside of the combustion housing 2 can be viewed from thefront of the front panel 1 e, and when the burner 3 is burned, such animpression as if the false firewood 4 is really burning can be visuallygiven.

The front panel 1 e can also be formed so that the peripheral portion ofthe window 1 h closely contacts the peripheral portion of the front faceof the combustion housing 2, but incidentally in the present embodiment,the front panel 1 e is arranged so that the window 1 h is slightlyseparated from the front face of the combustion housing 2, and a windowglass 1 i is fitted in the window 1 h. In addition, the above describedglass plate 2 b of the front face of the combustion housing 2 is fittedin the front door 2 c which is provided on the front face of the mainbody of the combustion housing 2 so as to be freely opened and closed,and the burner 3 in the combustion housing 2 can be provided withmaintenance in a state in which the front door 2 c is opened. This frontdoor 2 c is supported by the combustion housing 2 at the lower end parttiltably in a forward and backward direction, and is pushed backward bypushing means 2 e arranged on the upper wall 2 d of the combustionhousing 2 to closely contact the front face of the main body of thecombustion housing 2. A cover 2 f for covering the pushing means 2 efrom above is provided on the upper wall 2 d of the combustion housing2.

An air supply duct 5 which constitutes an air supply path is connectedto the bottom face of the combustion housing 2. Outdoor air is suppliedto the air supply duct 5 through an air supply extension tube which isnot shown. The outdoor air is forcibly supplied into the combustionhousing 2 through the air supply duct 5 by a suction force of acombustion fan 7 which will be described later.

In the outer case 1, a main heat exchanger 6 is arranged which ispositioned in the back side of the combustion housing 2 and is providedon the way of an exhaust path for exhausting a combusted gas sent fromthe combustion housing 2. The main heat exchanger 6 is connected to thecombustion housing 2 through a communication tube 6 a. The combustionfan 7 for forced draft direct ventilation, which is arranged in thefirst extension case 1 f, is provided on the way of the exhaust path. Anexhaust duct 8 which is connected to the downstream side of the mainheat exchanger 6 though the combustion fan 7 is inserted into the outercase 1 in a side space between the side plate 1 b in one side in atransverse direction of the outer case 1 and the combustion housing 2.

In addition, a control substrate 9 is arranged in one side in atransverse direction of the lower part of the inside of the outer case1, and a valve unit 10 for the burner 3 is arranged in a side spacebetween the side plate 1 b in the other side in a transverse directionof the outer case 1 and the combustion housing 2.

An air intake 11 is opened in the lower part of the front panel 1 e ofthe outer case 1, and a vent hole 12 is opened in the upper partthereof. A warm air fan 13 that takes in room air from the air intake11, passes the air through a section in which the main heat exchanger 6is arranged and sends the air to the inside of the room from the venthole 12 is arranged in the outer case 1, and thereby a warm air heatingfunction can be obtained.

The warm air fan 13 is arranged in a horizontally long fan box 14 whichis installed in the lower space of the combustion housing 2. The fan box14 has openings in both side parts of itself in a transverse direction.As is illustrated in FIG. 2, a wind introduction path 15 is providedwhich leads one part of air taken in from the air intake 11 to anaperture in one side in a transverse direction of the fan box 14 througha section in which the control substrate 9 is arranged and a section inwhich a fan motor 7 a for the combustion fan 7 is arranged. The windintroduction path 15 is constituted by: a first air guide plate 15 awhich surrounds the section in which the control substrate 9 is arrangedfrom the inner side in the transverse direction to the upper side; afirst air hole 15 b which is formed in the back plate 1 c of the outercase 1 so as to face a part surrounded by the first air guide plate 15a; a second air guide plate 15 c placed in the first extension case 1 f,which leads the air sent from the first air hole 15 b to the section inwhich the fan motor 7 a is arranged; and a second air hole 15 d which isformed in the back plate 1 c of the outer case 1 so as to be positionedabove the first air guide plate 15 a. Then, the air which is returned tothe inside of the outer case 1 from the second air hole 15 d is led tothe aperture in one side in a transverse direction of the fan box 14through a gap between the first air guide plate 15 a and the side wallin one side in a transverse direction of the combustion housing 2.

A pair of the warm air fans 13 are provided with a space therebetween ina transverse direction. A common fan motor 13 a which drives both of thewarm air fans 13 and 13 is arranged between both of the warm air fans 13and 13.

The room air which is blown from the warm air fan 13 is led to the venthole 12 through the section in which the main heat exchanger 6 isarranged, and a space between the upper plate 1 d of the outer case 1and the upper wall 2 d of the combustion housing 2. Then, the room airis heated by the main heat exchanger 6 due to the heat exchange with thecombusted gas, is heated to be a warm air, and is sent to the inside ofthe room from the vent hole 12. A louver 12 a which is tilted obliquelydownward is attached to the vent hole 12. Therefore, the warm air doesnot hit a face of a user who is warming oneself while sitting on a sofaor the like in front of a room heater.

Here, the space between the upper plate 1 d of the outer case 1 and theupper wall 2 d of the combustion housing 2 is partitioned into two roomsof an upper room 16 a and a lower room 16 b by a partitioning plate 16.The room air having passed through the section in which the main heatexchanger 6 is arranged is led to the vent hole 12 through the lowerroom 16 b. That is, a first air flow passage according to the presentinvention is constituted by a space between the back face of thecombustion housing 2 having the main heat exchanger 6 arranged thereinand the back plate 1 c, and the lower room 16 b. The first air flowpassage results in being formed along the outer surface of thecombustion housing 2.

In the upper room 16 a, a supplementary heat exchanger 6′ is provided onthe way of the exhaust path so as to be positioned in a downstream sideof the main heat exchanger 6. The supplementary heat exchanger 6′ isstructured so as to meander in a U-shape the exhaust duct 8 in atransverse direction. The back plate 1 c of the outer (case 1 has alower aperture 1 j which makes one part of the room air blown from thewarm air fan 13 flow into a second extension case 1 g through aflow-dividing plate 17, and an upper aperture 1 k which communicates aspace in the second extension case 1 g to the upper room 16 a. Thus, onepart of the room air blown from the warm air fan 13 does not passthrough the section in which the main heat exchanger 6 is arranged butis led to the vent hole 12 through the inside of the second extensioncase 1 g and the upper room 16 a. That is, the second air flow passageaccording to the present invention is constituted by the secondextension case 1 g and the upper room 16 a having the supplementary heatexchanger 6′ arranged therein.

In addition, a drain dish 18 is arranged on the upper face of the upperwall 2 d of the combustion housing 2. A drain pipe 18 a which leads tothe drain dish 18 is branched from and connected to a part of theexhaust duct 8 which is the bottommost part of the supplementary heatexchanger 6′. An exhaust extension tube 19 (see FIG. 2) for exhaustingthe combusted gas to the outdoors is connected to the downstream end(downstream end of exhaust path) of the exhaust duct 8. A drain formedin the exhaust extension tube 19 flows down to the exhaust duct 8, andflows into the drain dish 18 through the drain pipe 18 a.

In the present embodiment, a first damper 20 is provided so as to bepositioned in a gap between a lower end of the back face of thecombustion housing 2 and the flow-dividing plate 17 and can freely blockthe flow of the room air flowing into the first air flow passage by thewarm air fan 13, and a second damper 21 is also provided so as to bepositioned in the lower aperture 1 j of the back plate 1 c and canfreely block the flow of the room air flowing into the second air flowpassage by the warm air fan 13.

Next, the control in a period of a temperature-control operation whichvaries a combustion speed number (number that shows combustion amount ofburner 3) according to the deviation of the room temperature from theset heating temperature will now be described with reference to FIG. 5.In the description, the combustion speed number can be freely varied ina range between the seventh speed which is the upper limit and the firstspeed which is the lower limit, and a warm air fan speed number (numberthat shows rotation speed of warm air fan 13) can be freely varied in arange between the fourth speed which is the highest speed and the firstspeed which is the lowest speed. In addition, the rotation speed of thecombustion fan 7 can be varied according to the combustion speed number.

When the deviation of the room temperature from the set heatingtemperature is large and the combustion speed number becomes the fifthspeed or more (STEP 1), the room heater sets the warm air fan speednumber at the fourth speed, opens both of the first and second dampers20 and 21, and passes the room air into both of the first and second airflow passages (STEP 2). Thereby, the room air passed into the first airflow passage is heated by the main heat exchanger 6, and the room airpassed into the second air flow passage is heated by the supplementaryheat exchanger 6′. Here, the room air passed into the second air flowpassage flows to a section in which the supplementary heat exchanger 6′is arranged, in a state of being kept at a low temperature without beingheated by the main heat exchanger 6, and is efficiently heated by thesupplementary heat exchanger 6′. Accordingly, the heat exchangeefficiency of the whole room heater is enhanced as much as possible. Inaddition, when the combustion speed number is the second speed or morebut the fourth speed or less (STEP 3), the room heater sets the warm airfan speed number at the third speed in a state of having opened both ofthe first and second dampers 20 and 21 (STEP 4).

When the combustion speed number becomes the first speed, the roomheater judges whether the deviation of the room temperature detected bya temperature sensor (not shown) arranged in the vicinity of an airintake 11 from the set heating temperature is less than a predeterminedvalue YT1 or not (STEP 5). As a result, when the deviation is YT1 ormore, the room heater sets the warm air fan speed number at the secondspeed, closes the second damper 21 as is illustrated by an imaginaryline in FIG. 4, in a state of keeping the first damper 20 opened, andblocks the flow of the room air flowing into the second air flow passage(STEP 6). On the other hand, when the deviation is less than YT1, theroom heater opens the second damper 21 in a state of keeping the warmair fan speed number at the second speed to pass the room air into thesecond air flow passage, and also closes the first damper 20 as isillustrated by an imaginary line in FIG. 4 to block the flow of the roomair flowing into the first air flow passage (STEP 7).

At this time, when the room air is passed into both of the first andsecond air flow passages in a state in which the combustion speed numberbecomes the first speed, the temperature of the combusted gas flowing inthe exhaust extension tube 19 is excessively lowered, and the amount offormed drain increases. Then, in order to prevent the drain fromoverflowing from the drain dish 18, there is no other choice but toshorten the length of the exhaust extension tube 19. In contrast tothis, in the present embodiment, when the combustion speed numberbecomes the first speed, if the deviation of the room temperature fromthe set heating temperature is YT1 or more, the flow of the room airflowing into the second air flow passage is blocked as is describedabove, and the room air is heated only by the main heat exchanger 6.Accordingly, the temperature of the combusted gas flowing in the exhaustextension tube 19 can be prevented from being excessively lowered whilea heating amount necessary for promptly raising the room temperature tothe set heating temperature is maintained. As a result, even if theexhaust extension tube 19 is comparatively long, the drain from thedrain dish 18 can be prevented from overflowing. In addition, when thedeviation of the room temperature from the set heating temperature isless than YT1, the room air is heated only by the supplementary heatexchanger 6′, and the heating amount decreases. Therefore, the roomtemperature is inhibited from overshooting to a temperature over the setheating temperature, which enhances the temperature control performance.

By the way, when a heating load is light, the room temperatureoccasionally continues rising even though the combustion speed number isset at the first speed. Then, when the room temperature exceeds apredetermined upper limit temperature which is set higher than a setheating temperature (set heating temperature +3° C., for instance) thetemperature-control OFF operation is conducted. In thetemperature-control OFF operation, as is illustrated in FIG. 6, only apilot burner 3 a is burnt, the combustion in the burner 3 is stopped,and the warm air fan speed number is set at the first speed (STEP 100).

Here, when the temperature-control OFF operation is started, a warm airis sent from the vent hole 12 because heat is accumulated in the roomheater, but the heat in the room heater is released as time passes, thetemperature of air sent from the vent hole 12 is lowered, and acool-felt air starts to be sent. In this case, the timing at which thecool-felt air starts to be sent can be delayed by blocking the flow ofthe room air flowing into the first air flow passage or into the secondair flow passage in the temperature-control OFF operation period,because the heat in the room heater is slowly released.

Then, the room heater according to the present embodiment judges whetherthe temperature of the combustion housing 2 (temperature detected by atemperature sensor (not shown) which is installed in upper wall 2 d ofcombustion housing 2, for instance) is lower than a predeterminedtemperature YT2 or not (STEP 101), in the temperature-control OFFoperation period; when the temperature of the combustion housing 2 isYT2 or higher, opens the first damper 20 and also closes the seconddamper 21 (STEP 102); and when the temperature of the combustion housing2 is lower than YT2, closes the first damper 20 and also opens thesecond damper 21 (STEP 103).

According to the room heater, when the temperature of the combustionhousing 2 is YT2 or higher, the room air is passed only into the firstair flow passage, heat is released from the main heat exchanger 6, andalso from the combustion housing 2 facing the first air flow passage.When the temperature of the combustion housing 2 is lowered by this heatdissipation and has reached a temperature lower than YT2, the room airis passed only into the second air flow passage, and heat is releasedfrom the supplementary heat exchanger 6′. Therefore, the timing at whicha cool-felt air starts to be sent can be delayed as much as possible.

In the above, the embodiment of the present invention is described withreference to the drawings, but the present invention is not limitedthereto. For instance, in the above described embodiment, the firstdamper 20 and the second damper 21 are provided, but any one of both ofthe dampers 20 and 21 can be omitted as well. In addition, in the abovedescribed embodiment, the combustion fan 7 is provided on the way of theexhaust path, but the combustion fan can be provided on the way of theair supply path as well.

Furthermore, in the above described embodiment, the vent hole 12 isopened in the upper part of the front panel 1 e of the outer case 1, andthe warm air fan 13 is arranged in the lower part of the combustionhousing 2. However, the vent hole 12 may be opened in the lower part ofthe front panel 1 e, and the warm air fan 13 may be arranged in theupper part of the combustion housing 2. In this case, the supplementaryheat exchanger 6′ may be arranged in the second extension case 1 g, andthe drain dish 18 may be arranged in the lower part of the combustionhousing 2. In addition, in the above described embodiment, the presentinvention is applied to a fireplace type of a forced draft direct venttype room heater, of which the inside of the combustion housing 2 can beviewed from the front of the front panel 1 e, but can be also applied toa forced draft direct vent type room heater of which the inside of thecombustion housing cannot be viewed.

1. A forced draft direct vent type room heater comprising: an outercase, a combustion housing which is provided in the outer case andhouses a burner therein, a combustion fan for forced draft directventilation, which is provided on the way of any one of an air supplypath that supplies air for combustion to the combustion housing and anexhaust path that exhausts a combusted gas from the combustion housing,a main heat exchanger in an upstream side and a supplementary heatexchanger in a downstream side, which are provided in the outer case andare provided on the way of the exhaust path, an air intake and a venthole which are opened in the outer case, a warm air fan in the outercase for taking in room air from the air intake and sending the air tothe inside of the room from the vent hole through a section in which themain heat exchanger and the supplementary heat exchanger are arranged,and a drain dish which is provided in the outer case and which makes adrain formed in an exhaust extension tube that is connected to adownstream end of the exhaust path flow into the drain dish itself,wherein a first air flow passage that leads to the vent hole in theouter case through a section in which the main heat exchanger isarranged and a second air flow passage that does not pass through thesection in which the main heat exchanger is arranged but is led to thevent hole through a section in which the supplementary heat exchanger isarranged are separately formed in the outer case, at least one of afirst damper which can freely block the flow of the room air flowinginto the first air flow passage due to the warm air fan and a seconddamper which can freely block the flow of the room air flowing into thesecond air flow passage due to the warm air fan is provided, and theflow of the room air flowing into the first air flow passage or thesecond air flow passage is blocked, in a weak combustion period in whicha combustion amount of the burner is a predetermined amount or less. 2.The forced draft direct vent type room heater according to claim 1,wherein when a room temperature exceeds a predetermined upper Limittemperature which is set higher than a set heating temperature, atemperature-control OFF operation is conducted in which the warm air fanis rotated at a low speed in a state that the burner stops combustion,and in the temperature-control OFF operation period, the flow of theroom air flowing into the first air flow passage or the second air flowpassage is blocked.
 3. The forced draft direct vent type room heateraccording to claim 2, wherein both of the first damper and the seconddamper are provided therein, the first air flow passage is formed alongthe outer surface of the combustion housing, and when the temperature ofthe combustion housing is a predetermined temperature or higher in thetemperature-control OFF operation period, the room air is passed intothe first air flow passage and the flow of the room air flowing into thesecond air flow passage is blocked, and when the temperature of thecombustion housing is lower than the predetermined temperature, the roomair is passed into the second air flow passage and the flow of the roomair flowing into the first air flow passage is blocked.
 4. The forceddraft direct vent type room heater according to claim 1, wherein both ofthe first damper and the second damper are provided therein, and whenthe combustion amount of the burner decreases to a predetermined lowerlimit combustion amount, in a temperature-control operation period ofvarying a combustion amount of the burner according to a deviation ofthe set heating temperature from the room temperature, if the deviationis a predetermined value or more, the room air is passed into the firstair flow passage and the flow of the room air flowing into the secondair flow passage is blocked, and if the deviation was less than thepredetermined value, the room air is passed into the second air flowpassage and the flow of the room air flowing into the first air flowpassage is blocked.
 5. The forced draft direct vent type room heateraccording to claim 4, wherein when a room temperature exceeds apredetermined upper limit temperature which is set higher than the setheating temperature, the temperature-control OFF operation is conductedwhich rotates the warm air fan at a low speed in a state in which theburner stops combustion, and in the temperature-control OFF operationperiod, the flow of the room air flowing into the first air flow passageor the second air flow passage is blocked.
 6. The forced draft directvent type room heater according to claim 5, wherein both of the firstdamper and the second damper are provided therein, the first air flowpassage is formed along the outer surface of the combustion housing, andwhen the temperature of the combustion housing is a predeterminedtemperature or higher in the temperature-control OFF operation period,the room air is passed into the first air flow passage and the flow ofthe room air flowing into the second air flow passage is blocked, andwhen the temperature of the combustion housing is lower than thepredetermined temperature, the room air is passed into the second airflow passage and the flow of the room air flowing into the first airflow passage is blocked.